Corticosteroid Binder IB, A Potential Second Glucocorticoid Receptor
In 1973 (Litwack et al. 1973) we discovered a second liver glucocorticoid binding protein separable from transcortin and from the glucocorticoid receptor that we named binder II. We named the new protein IB. The nomenclature developed from the sequence of elution of glucocorticoid or metabolite binding proteins from DEAE—Sephadex columns at pH 7.5. Since ligandin, a steroid metabolite binding protein eluted first (pI ~8.9), it was named IA and the new binding protein was eluted just after it, hence IB. The receptor eluted next (II) and subsequently other proteins including Transcortin (IV) were eluted. Initially we thought, since IB was a binder of unmetabolized potent glucocorticoids, it might function as a “storage” protein in the cytosol, perhaps analogously to the cytosolic thyroid hormone binding protein or that it might be a second receptor (Litwack and Rosenfield 1975). Recently, there has been much emphasis on the proteolytic degradation of the glucocorticoid receptor into forms that retain the steroid-binding function and either retain or do not retain the DNA-binding site (Wrange and Gustafsson 1978; Sherman et al. 1979). In this paper we review and extend the information on corticosteroid binder IB. We try to draw some conclusions about its possible function in light of the recent emphasis on artifactual proteolytic digestion products of the glucocorticoid receptor.
KeywordsGlucocorticoid Receptor Triamcinolone Acetonide Liver Cytosol Chloromethyl Ketone Proteolytic Inhibitor
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